Water pollution due to organic pollution had been a major worldwide environmental issue, resulting in the development of biological carbon removal process, also known as secondary treatment, in the early 20th century. Although the focus of sanitary researchers then shifted to the development of biological nutrient removal processes to tackle eutrophication problems, with many authorities setting guidelines and standards for controlling discharges of nitrogen and phosphorus to sensitive waters, the merits of secondary treatment cannot be overlooked. These include a shorter sludge age, a smaller reactor, simpler operations, and less foaming problems.
USEPA (1984) defines secondary treatment as having a discharge with 30-day average CBOD5 and SS of 25 mg/L and 30 mg/L, respectively. With these operational and economic benefits, secondary treatment is still one of the most popular sewage treatment processes in the world, in particular, for discharges leading to less sensitive waters, e.g., oceanic waters, as well as for water reclamation.
One of the common applications for secondary treatment is the production of reclaimed water for irrigation. Irrigation accounts for 70% of the water consumption in the world. To conserve precious water resources, many cities have adopted a dual water supply system supplying freshwater and reclaimed water for various uses, such as crop and landscape irrigation. The minimum treatment level required for irrigation is secondary treatment plus disinfection. For the purpose of irrigation, secondary treatment is superior to biological nutrient removal as it not only costs less, but also retains nutrients as essential fertilizers.
Since the introduction of the activated sludge process in 1914, the biological process for organic carbon removal from wastewater in secondary treatment has remained the same for almost a decade. FIG. 1 is a schematic diagram depicting a conventional biological sewage treatment process for achieving carbon removal using a heterotrophic oxidation reactor. The process involves the heterotrophic oxidation of organic carbon to CO2 and the conversion of the remaining organic carbon into sludge for removal at the secondary sedimentation tank.
To control eutrophication, nitrogen removal is necessary. The development of Biological Nitrogen Removal processes in the 1960s modified the process by introducing autotrophic nitrification and heterotrophic denitrification steps to the treatment process. FIG. 2 is a schematic diagram depicting a conventional biological sewage treatment process for achieving carbon and nitrogen removal using heterotrophic denitrification and autotrophic nitrification reactors. Since heterotrophic carbon oxidation and denitrification process has a high sludge yield factor, excess sludge wastage, handling and disposal from these processes are required.
Many countries have been relying on the reuse of treated sewage for providing water for various types of applications such as street and car washing, toilet flushing, landscape irrigation, environmental water, and groundwater replenishment. FIG. 3 is a diagram showing the operation of a typical sewage treatment and water reclamation plant. Generally, the minimum treatment for these types of water reuses is biological secondary treatment followed by a water reclamation plant involving sand/membrane filtration and disinfection. Nitrogen removal, i.e., nitrification and denitrification, can be provided if necessary.
The application of the water reuse system, however, is quite difficult in areas where seawater is used for toilet flushing as a means of water conservation, such as Hong Kong. This is because when seawater (containing a salt concentration of about 35,000 mg/L) was used to flush the toilets, and the sewage generated will become saline with a salt concentration of about 7000-10,000 mg/L. This high level of salt affects several types of water reuse options such as irrigation or groundwater replenishment. Nevertheless, as seawater also contains about 2600 mg/of sulfate, it provides the necessary sulfate ion as the electron carrier for the development of a new type of wastewater treatment process. This use of sulfate characterizes the SANI process.